Anatomy of a Profile

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This reserved article originally appeared in CHROMiX ColorNews Issue 54 on February 27, 2014.

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Here's a basic primer for those wanting to know just what an ICC profile is. I've tried to tuck in all the basic little tidbits of information and color management axioms that may be useful in your color management workflows. See Steve Upton's article on The Color of Toast to see a light-hearted analogy for understanding color management.

Contents

It's a computer file

An ICC profile is a file that resides on your computer, much as other files do. One thing to keep in mind is that every profile has an internal name as well as an external name. The file name is what you will see when looking through your files on your computer using Windows Explorer, or a Finder window on a Mac. You can rename this file as you would any other file on your computer. Watch out though! The internal file name is the name that is listed when you are viewing a list of profiles from inside an app like Photoshop or Lightroom. When the ICC profile format was created in the olden days, some operating systems had a limit to how many characters a file had. So the internal file name allowed for more characters in order to describe a profile better. While it's a generally accepted practice to keep these external and internal names identical, it's important to recognize that these can be different. This might explain your confusion one day, when you have renamed your output profile to something like "My favorite profile.icc" and then go looking for it in your profile drop-down box. You won't find it there because it will still be named "Ep3880 Premium Luster.icc" - the internal file name. If you are on a Mac, the ColorSync Utility will allow you to change the internal profile name. Windows has no built-in program that can do this, so you would need a 3rd-party program like ColorThink v2 or ColorThink Pro to change the internal name.


It's a snapshot in time

When a device profile is created, it has captured the device's behavior at a certain point in time. With a printer profile this was done using certain inks, paper and printer - and certain settings in the software such as media type, linearization and ink limiting. It's always good to keep in mind that if any of these processes change in any way, the profile become invalid or obsolete. It no longer reflects accurately the state of that process. If you're suddenly getting different color, check to see what's changed.


It connects between color spaces

CIE Lab is a universally recognized 3D space for defining a color precisely. If you're looking at a Lab value, then you have there an actual defined color. You can think of Lab as a sort of universal translator for color. A profile figures out how best to translate your device's numbers into the "universal color language": Lab - and also how to go the other direction from Lab into device space. So if you are printing to your printer out of Photoshop, a simplified way to visualize this is: Your image starts out in your AdobeRGB working space numbers, > then the AdobeRGB profile transforms it into Lab space, > your Premium Luster profile picks it up in Lab space, > and transforms the image into Epson3880 printer space numbers.

Notice that this involves two profiles. Profiles are almost always used in pairs. You're converting out of a device's space into the universal translator, and then out of the universal translator into the new device's space.

Printing (rendering) direction of a profile
Proofing direction of a profile

It goes in two directions

A printer profile will work in two directions. A typical use for a profile is in sending an image to the printer. This is called the rendering direction or the B to A direction. It handles the job of taking color numbers from the Lab space and converting them to numbers that the output device (the printer) will use to put the appropriate amount of ink onto the paper. This is Lab to Printer. A profile can also be used in the opposite direction, from Printer to Lab. This is the direction that will be used when you use a profile to soft-proof in Photoshop. When soft-proofing, you are asking the profile to convert from the printer space into Lab space so that you can see the results on the screen in Photoshop. This is called the proofing direction or the A to B direction. You can think of this as using the profile to "print" to your monitor. These two directions are saved in different tags inside the profile. These tags are also called tables or LUTs (Look-Up-Tables).

Some profile creation software like X-Rite's i1Profiler allow you to choose different sizes for these separate sets of tables. This might be useful if you are primarily concerned with printing and not with proofing. Choosing a smaller table size for the A to B direction will allow your profile file size to be smaller without sacrificing any quality in your printing.

It's possible to swap these tables around between profiles. If you like the printing result of one profile, but prefer the soft-proofing behavior of a different profile, some programs like ColorThink will allow you to put a table from one profile into another.

Tag table

Embedded in a profile is a tag table that contains the rendering intent tag tables (more on those below), a copyright tag (so you can find out who made the profile), tags that define the white point, black point and primary colors of the profile, and sometimes even the measurement data and parameters that were used to construct the profile originally.


An intense look at Intents

Also included in the guts of a profile are four different rendering intents. The program you're printing from will allow you to choose one of these intents, and you can choose according to your intentions for how you want your colors handled. Your rendering intent choice will affect in a big way how any out of gamut colors will be handled.

Perceptual

Some people will want to preserve the relationship between colors that are close to each other, so any color changing will appear more "natural." or evenly spaced. This is great for portrait work. Facial colors will tend to have smooth gradients from one shade to another. Perceptual will tend to de-saturate colors though, so this is not so good for vivid scenery images. Perceptual will also tend to give you more shadow detail. Rel Col. and Abs. Col. are constructed along more or less mathematical lines. But creating good Perceptual and Saturation rendering intents is almost an art form. It's a bit tricky to preserve the differences between colors in a natural way and at the same time try to not de-saturate the colors too much when brought into gamut. You get different results with different profile-building software.

Relative Colorimetric

For your eye-popping, maximum saturation images, you will want to use Rel Col. It will preserve as much gamut as it can, and it will only desaturate your out-of-gamut colors to just within your printer's ability to print. This will also mean that there might not be much difference between various shades of saturated pixels. A red fire engine will tend to blend into one blob of the same maximum red. In Photoshop, you can select "Black Point Compensation" in order to get the same preservation of shadow detail in Rel. Col. that you can get with Perceptual. Rel. Col. is a good choice for Fine Art reproduction work, as it will give you the most accuracy for colors that are already in-gamut for your printer. This is also a good all-around rendering intent for a printer with a wide gamut, which is expected to contain most of the colors of your images.

Absolute Colorimetric

This is rarely used unless someone wants to maintain the same whiteness on the new print as you had on the profile you're converting from. If you're printing a hard proof to simulate what a newspaper print will look like, you can use Abs Col while converting from a newsprint profile to an inkjet profile, and your inkjet print will have all the gray whites that a newsprint paper would have.

Saturation

This is very similar to Perceptual, but is intended to offer slightly more saturation than Perceptual normally offers. This is rarely used, but read this note for some inspiration to try this rendering intent more often:


It is important to note that all ICC printer profiles have these four rendering intents built into them. It's part of the ICC specification. There is apparently a myth floating around out there that there is only one rendering intent, say perceptual, in the profile - or that the profile when it was made, was intended for only a certain rendering intent. Not true! I'll say it again: All ICC printer profiles have these four rendering intents built into them. People do have different goals in mind when printing, and all four of these are very valid on different occasions. A profile is not made for "one" rendering intent. You usually just need to click a drop down box and choose a different one if you wish.

Sometimes a paper manufacturer's instructions about how to print will include generic recommendations, and may suggest you use the Perceptual rendering intent. This intent is sometimes considered the "safest" rendering intent to recommend because this allows our perception of the differences between colors to continue to look natural, even if the overall impression may be a bit "flat". If I had no idea what your images were like, and no idea of what printer you were using, I guess I'd recommend Perceptual too. When a profile is created, it is required to have a "default rendering intent". This is just an information field in the header of the file, and has no effect on the creation or function of the profile. (It's not even picked up or used by most programs.) This is often set at Perceptual, but that does not mean this was the rendering intent the profile was "made for". It is understandable why Perceptual is recommended as a place to start. But don't let that stop you from trying out the others!

More ideas

If you ever have any doubt that a profile is doing its job, or to put it another way - if you ever want to test your workflow to see if it is applying your profiles correctly, check out our article on Stunt Profiles. These are artificially constructed profiles that will help you test whether rendering intents are being used correctly, or if your monitor profile is actually being picked up and used, or if you just like printing strange pictures.


I have mainly concentrated on the use of the typical output profile for use when printing. For information on the other types of profiles, see this newsletter article: http://www.colorwiki.com/wiki/ColorNews_No._22_-_The_7_ICC_profile_types

Thanks for reading,



Patrick Herold

CHROMiX

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